The basic use cases for pub.file services would cover i/o from your file system. You can check the pub.file services documentation which includes services for creating files, deleting files etc. (the CRUD).
For data received over the network, you may want to check public services such as pub.client:http which is in the pub.client folder.
Please clarify what you mean by converting files to XML format, is the incoming data in a format such as JSON and you need to convert this to XML? There are services in the pub.json and pub.xml folders which may be of help.
A general note is that it would help to have a better understanding of service designer, since flow services are written usually in designer. Please refer section 3 of webMethods Service Designer – First steps, tutorials & How-tos for tutorials and articles.
Good luck exploring!
-NP